1. Field of the Invention
The present invention relates to a planetary gear device of an automatic transmission used in a motor vehicle, for example.
2. Description of Prior Art
A known example of the planetary gear device of the automatic transmission is disclosed in Laid-open Publication No. 2-50550 of Japanese Utility Model Application. The known planetary gear device of the automatic transmission includes a sun gear, a plurality of pinion gears that engage with the sun gear, a carrier that rotatably supports the pinion gears, and a ring gear having inner teeth formed on the inner periphery thereof for engagement with the pinion gears. A spline is formed on the outer periphery of the ring gear such that the spline engages with drive-side friction plates (drive plates) of a clutch.
In the known planetary gear device as described above, however, the spline is formed over the entire width of the outer periphery of the ring gear, and therefore the smallest thickness or root diameter of the ring gear is reduced, resulting in a reduced strength of the ring gear.
The strength of the ring gear may be increased by increasing the thickness of the ring gear (namely, by increasing the root diameter of the spline formed on the outer periphery of the ring gear). If the root diameter is increased without changing the outside diameter of the spline, however, contact areas between the friction plates of the clutch and the spline engaging with these plates are reduced, resulting in an increased bearing pressure or stresses on the friction plates and the spline.